This invention relates to windshield wipers, and more particularly to an improved aircraft windshield wiper arm assembly particularly well suited for use with curved aircraft windshields.
Larger aircraft (both rotary and fixed wing) in commercial, general, and military aviation are required to have external windshield wipers for aiding visibility of the flight crew by clearing rain, snow, condensation, etc., from the outer surface of the aircraft's windshield. Typically, aircraft windshield wiper systems include a motor or drive for oscillating one or more wiper arms through an arc across the outer surface of the windshield. Typically, an elastomeric wiper blade is carried by the wiper arm in such manner that the arm applies a relatively linear force along the length of the wiper blade so that the latter uniformly scrapes moisture and dirt from the outer surface of the windshield with a squeegee-like action.
In aircraft windshield wiper applications, it is necessary that the wiper arm and blade be resiliently held in its parked position with the elastomeric blade bearing on the windshield while the aircraft is flown through its entire flight envelope. This includes the requirement of holding the blade down on the windshield even at extremely high air speeds when the windshield wiper would not normally be operated. Of course, the wiper blade must be resiliently held in contact with the outer surface of the windshield as it operates under various flight conditions. As the wiper operates, the wiper arm and the blade present themselves differently to the air stream at different positions of the operational cycle of the wiper, and thus encounter a variety of aerodynamic forces tending possibly to lift the blade from the windshield at one point of its operational cycle and possibly, at another point of its operational cycle, to force the windshield blade against the windshield. Typically, the wiper arm is spring loaded so as to resiliently bias the blade against the windshield with sufficient force so as to firmly maintain the blade in engagement with the windshield under all operating conditions.
On certain aircraft, such as many commercial jet liners, the windshield panels are flat. Thus, with a given resilient preload or spring force on the wiper arm, the biasing force exerted on the wiper blade by the arm as the wiper moves across the windshield remains relatively constant.
However, on other aircraft with curved windshields, the wiper arm must deflect so as to permit the blade to follow the curved contour of the windshield as the wiper arm sweeps across the windshield. This flexing of the wiper arm has two adverse effects. First, as the wiper arm oscillates (sweeps) across the curved windshield, it moves toward and away from its attachment point to the aircraft so that the biasing force applied to the blade changes markedly. In order to ensure that an adequate biasing force is maintained on the blade even when the wiper arm is in its most deflected position, the wiper arm is often initially biased to a very high level so as to compensate for biasing force drop off. Secondly, as the wiper arm oscillates, the alternating changes in the biasing force on the wiper arm coupled with the higher initial biasing force has resulted in fatigue failures of many of the prior art wiper arms and/or preload adjustment mechanisms used on curved windshields. In certain aircraft applications, preventive maintenance requires wiper arms to be changed as often as every fifty hours of flight time. Not only does this relatively short replacement life constitute undue expense, but an unexpected failure of a wiper blade assembly could, under certain flight conditions, impair the flight crew's vision and thus place an extra burden on the flight crew in adverse weather conditions.
Among the several objects and features of the present invention, may be noted the provision of a wiper arm particularly well suited for use on a curved windshield which maintains an acceptable biasing or preload force on the wiper blade as the blade oscillates or sweeps across the curved windshield;
The provision of such a wiper arm which has markedly improved service life and thus will have markedly lower maintenance costs;
The provision of such a wiper arm which may be retrofitted on existing aircraft without modification to the aircraft or to the wiper drive system;
The provision of such a wiper arm in which the biasing force on the arm may be readily adjusted to a predetermined level; and
The provision of such a wiper arm which is of light weight and economical construction.
Other objects and features will be in part apparent and in part pointed out hereinafter.